The Old Etonian spent his Fresher year rowing for his college and failing in his chosen subject, Law, in which he was one of Cambridge’s least promising students.

And then came an evening, October 12th 1975, about a week into his second year, that changed everything.

His friend Nicky Hills, a Christian student at Trinity, invited Welby along to his room to eat and talk God. At about quarter to midnight, Hills upped the ante.

He told Welby: “Jesus died on the cross for you, Justin.” At that moment, the Old Etonian says the penny dropped and, as he later put it, “I asked Jesus to be the Lord of my life”.

He says the sense that something had changed was “instantaneous” and once told an interviewer that his conversion felt, “like the world changing, like someone I’d never known coming into the room and being there”.

But Welby’s dramatic conversion wasn’t a one off. It was the work of a powerful group of evangelical Christians called the Cambridge Inter-Collegiate Christian Union, or CICCU, and it happened to many impressionable public schoolboys who arrived at Cambridge during that period.

A few years ago I tried to speak to people who knew Welby at Cambridge, including the former Daily Telegraph editor Charles Moore and the cabinet minister Oliver Letwin, David Cameron’s most trusted advisor. I never wrote the profile of Welby that I planned, but what his peers said about the now-Archbishop’s university years is an interesting story in itself.

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In CICCU, and the evangelical group at Trinity College known as the “God Squad” by other students, Welby found a brand of conservative, preaching Christianity that he liked – and never left.

“CICCU certainly worked hard on freshmen,” says Charles Moore, who studied at Trinity at the same time. “In Welby’s case, I expect an Etonian would have been deputed to recruit him – [Nicky] Gumbel or [Nicky] Hills or Nicky Lee [Gumbel and Lee are now high-ranking evangelicals]. The business of finding God on a particular day is a long-standing evangelical phenomenon.”

Another contemporary of Welby’s who encountered CICCU’s young evangelists told me they were often successful with “uncomfortable, uneasy boys arriving in Cambridge”.

“I think they tried to be kind, but it was a very definite and convinced rightness about them,” says Bob Reiss, one of Trinity’s two chaplains at the time and later the Canon Treasurer at Westminster Abbey. Reiss recalls that Welby’s group of Christians at Trinity was “largely” made up of Old Etonians – the officer class of Cambridge’s growing evangelical movement at the time.

Professor Philip Allott, who taught Law at the college, refers to them as the “socially privileged evangelicals of that period,” and another of Trinity’s chaplains – Ralph Godsell – says they “approached their religion in the same way that you might think David Cameron approaches politics. It’s almost noblesse oblige – God will look favourably upon us because we are born to lead.”

What distinguished the evangelical group more than anything, though, was their social conservatism. “They would have all been firmly against homosexuality,” says Reiss, recalling a deep divide in Seventies Cambridge between a group representing gay Christians and those on the evangelical side. According to Reiss, the latter group would have said, “[it’s] completely wrong – if someone is gay they ought to be cured”.

Reiss says: “There were those of us who were much more tolerant and accepting of that [gay Christians] than would have been the case in the Christian Union group.” I have found no evidence that Welby, or anyone else mentioned in this article, held intolerant views.

Among the Trinity clique (three of whom were called Nicky) was Nicky Gumbel, a fellow Old Etonian whose popular Alpha Course programme has made him one of the country’s most influential Christians. Andrew Atherstone, who wrote a biography of Welby, describes how Gumbel appeared to fellow students at parties. “Instead of pulling the girls he had evangelistic tracts in his jacket pockets.”

Welby himself was known for not drinking at boozy end-of-term rowing dinners. His friend Charlie Arbuthnot, a fellow CICCU member, rowed in the Trinity boat which was coxed by Welby and therefore spent many hours sitting opposite him on the river. He says the two of them stood out as the only ones drinking water at the dinners.

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Welby’s father Gavin was an alcoholic who has been described as a “Walter Mitty character”. He changed his name and lived a high-society second life in America before marrying Welby’s mother Jane Portal (a former personal secretary of Winston Churchill).

In the few years before his death in 1979, he was looked after by his son – an experience which might have been every bit as important for the young Welby as his conversion Christianity.

“It was all very complicated,” the Archbishop has said. “I was his only relative. He had few friends.” Welby says it was 20 years before he could look at the period of his father’s death in his scrapbooks “without just finding it intolerably painful”.

Bob Reiss, then the chaplain at Trinity, told me he does not want to speak publicly about how he counselled the future Archbishop during the very difficult period before and after his father’s death, though he admits that he did.

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At the end of exams each year, Welby and his rarefied Christian group decamped to Bash Camps (named after evangelical cleric Eric “Bash” Nash) – “house parties in the summer where boys from the top public schools would go,” Atherstone explains – to be converted by Oxford and Cambridge boys who had already done so.

Just as his newfound Christianity furnished the future Archbishop with a new sense of direction, a conservative morality and a ready-made social group, it also found him a wife. In his third year, Welby was asked by a Christian friend in London to look after a new fresher, Caroline Eaton. He married her three years later in 1979.

What it didn’t bring him was grades. Welby once said in an interview that after Cambridge he “toddled off into the oil industry because I couldn’t get a job anywhere else” and joked that he was one of the few among his peers who wasn’t approached by M16. It’s possible that the spymasters simply missed the diminutive Old Etonian on their famous hiring missions to colleges like Trinity, but it is more likely that they looked at his academic record and wrote him off.

The official website of the See of Canterbury records that Welby “studied History and Law” in Cambridge, but a visit to the University Library yields a slightly different answer and explains why he spent four years at university, not three.

The class lists show that Welby studied Law on its own in his first year, and that he got a third. In his second year he persevered with the course, and again received a third, placing firmly in the bottom 10 per cent of students in his year – dropout territory that a Cambridge contemporary says was usually peopled by “the ones who appreciated the other things in life!”

Welby then switched to History, a move that would have been extremely difficult given his previous underperformance, and received a 2.ii in his third year and a 2.i in his fourth, behind the future minister Letwin and the future columnist Peter Oborne, also a contemporary, who both got firsts. Letwin was an obvious political star already, in a way that Welby wasn’t. (“I don’t think he stood out as one of the obviously outstanding people of his generation,” says Reiss. Did Letwin? “Yes.”)

Letwin himself more generous. He told me he remembers Welby as “a refreshingly clear-headed and engagingly intelligent man” and describes himself as “an admirer for several decades”. He added: “The Church is very lucky to have him.”

Oborne says he doesn’t remember Welby, but that the conclusion to draw from his poor grades and switch to History is clear: “It tells you something: that he did Law and hated it.”

Students who aren’t nailing their exams can take a bit of encouragement from the fact that someone who got a third in his first two years has gone on to lead the Anglican communion – an organisation of 85 million members.

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It’s odd how little has been written about Welby the student, given how transformative his years at Cambridge seem to have been. Lots of people have their lives changed at university, but not normally with this much consequence.

Those years in Cambridge might get more scrutiny in the coming months and years. In fact, we shouldn’t be surprised if someone takes a deeper look at the positions taken by Welby and his evangelical friends on the issues that threaten to tear his church apart – notably the role of women in Christianity, and gay rights. If anyone can find the CICCU pamphlets and meeting minutes from that time, the Archbishop may find them being quoted at him.